Mapping America’s access to nature, neighborhood by neighborhood

A city is a science experiment. What happens when we separate human beings from the environment in which they evolved? Can people be healthy without nature? The results have been bleak. Countless studies have shown that people who spend less time in nature die younger and suffer higher rates of mental and physical ailments.

“There’s a really, really strong case for proximity to nature influencing health in a really big way,” said Jared Hanley, the co-founder and CEO of NatureQuant, an Oregon start-up whose mission is to discover what kind of nature best supports human health, map where it is and persuade people to spend more time in it.

Using satellite imagery and data on dozens of factors — including air and noise pollution, park space, open water and tree canopy — NatureQuant has distilled the elements of health-supporting nature into a single variable called NatureScore. Aggregated to the level of Census tracts — roughly the size of a neighborhood — the data provide a high-resolution image of where nature is abundant and where it is lacking across the United States.

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Quantifying nature reveals unsettling truths — about how the densest neighborhoods are often bereft of nature, and about how the poorest city dwellers have the least access to the nature’s health benefits. But it could also help pinpoint which parts of our urban landscapes would benefit most from an infusion of nature.

Why does nature make us healthy? One answer is the “old friends hypothesis” that our immune systems grow stronger when regularly exposed to the natural pathogens with which we evolved. Or maybe it’s that being in nature nudges us to exercise and socialize. A third theory is that cities are just unhealthy, exposing us to lead, asbestos and vehicle exhaust, not to mention the stress of traffic and noisy construction.

Yet like space dust accreting to form planets, humans seem compelled to gather in cities. Two hundred years ago, just 7 percent of people in the United States lived in an urbanized area. By 1970, when Joni Mitchell was lamenting that we’d “paved paradise,” that figure was up to 75 percent. Today, 86 percent of us live in cities, and the share continues to rise.

The denser the city, the less health-supporting nature you are likely to find. Among the 500 most populous cities in the United States, Suffolk, Va., with just 147 people per square mile, has the highest NatureScore. Union City, N.J., is by far the densest — almost 30,000 people per square mile — and has one of the lowest NatureScores.

But density is not destiny. New York, for instance, has a better NatureScore than Los Angeles and Chicago, even though its population density is higher. And the best way to boost a city’s NatureScore is to plant trees.

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The Arbor Day Foundation, which directs millions of dollars to tree planting projects around the world, started using NatureQuant’s data in 2021. If a donor comes to the Arbor Day Foundation with a plan to plant trees in a posh part of town, the data can help make the argument that the trees would do more good elsewhere.

“Everybody wants to plant in their neighborhood,” said Jeff Salem, director of communications for the Arbor Day Foundation. “But this helps that conversation of, ‘Hey, you might live in North Chicago in a really great neighborhood, but really, as you can see here, there’s some neighborhoods on the South Side that really could use your support with trees.’”

In rural America, it doesn’t matter if you are rich or poor, Black or White, dropped out of high school or have a PhD: you are still likely to have access to health-supporting nature. But in cities, differences in access to nature are as stark as other forms of inequality.

For example, among the fifth of Census tracts with the lowest levels of education, the average NatureScore is just 37, compared with an average score of 68 in the most educated Census tracts. The Census tracts with the lowest share of White people have an average NatureScore of 45, compared with 73 in the tracts with highest share of Whites.

NatureScores can identify neighborhoods that need trees. Planting them is another matter. “We use [the data] as a starting point. But, you know, the devil is in the details,” said Christina Smith, the executive director of Groundwork Bridgeport, an environmental nonprofit in Bridgeport, Conn., where she grew up.

Suppose you want to boost a neighborhood’s NatureScore by lining the sidewalk with trees. Before you buy the first sapling, you need to make sure the sidewalk is wide enough to fit a tree and still comply with the Americans With Disabilities Act. And you’ll need money to hire workers or face the dangerous prospect of twenty high school volunteers packed into a narrow sidewalk with cars whizzing by.

What about just giving free trees to neighborhood residents? If most people rent, they won’t have the authority to plant on their property. If you manage to track down property owners, they might not want the burden of planting and caring for trees.

Yet Groundwork Bridgeport has figured out how to host successful tree giveaways (door knocking works better than direct mail, and it helps to have friends at community gardens). Last year, they distributed 100 trees to residents on the east side of the city. If all those trees are still around in 30 years, it will boost the area’s NatureScore by 15 points, amounting to an increase of a year of life expectancy for people in the neighborhood, NatureQuant told me.

Thirty years is a long time to wait, which is one of the reasons cash-strapped city governments often view tree planting as a frivolous expense compared to more pressing matters like crime and homelessness. “Trees are not a silver bullet. Trees are not going to solve poverty,” said Dan Lambe, the Arbor Day Foundation’s CEO.

But quantifying nature can make sure trees are going where they’re needed most, Lambe said: “We know we can make a difference in people’s lives by emphasizing neighborhoods that have been disadvantaged, who have been ignored, that just simply don’t have the tree benefits.”